This research stemmed from an authentic practice-based problem encountered by me as a Continuing Professional Learning (CPL) facilitator in the area of primary science: the challenge of transferring engaging CPL opportunities into actual classroom practice. Research highlights a number of concerns regarding the teaching and learning of primary science in Ireland, including a lack of teacher confidence and limited participation in CPL. There is a need for increased levels of Pedagogical Content Knowledge (PCK) and Subject Matter Knowledge (SMK) among teachers as well as greater use of Inquiry Based Science Education (IBSE) methodologies in the classroom and further development of children’s scientific skills. Lesson Study (LS) was identified as an effective method of CPL, although most LS research has been in the area of mathematics. This study explored the use of school-based LS as a means of developing primary teachers’ orientations towards the teaching of science, an overarching aspect of PCK in science. Furthermore, the study aimed to identify the features of school-based LS that are most effective in supporting the teaching of primary science. In addition, the role of the researcher as an emergent Internal Knowledgeable Other (IKO) was examined. Research was conducted from a social constructivist viewpoint, underpinned by social learning theory. An evaluative, instrumental, collective case study was utilised to collect data across three cycles of LS. Teacher orientations were analysed based on an emergent framework. The Value Creation Framework (Wenger-Trayner & Wenger Trayner, 2020) was used to identify the effective features of LS as a CPL methodology in primary science. Findings reveal that LS supported both the development of teachers’ orientations and SMK, which combined, positively impacted children’s learning. Findings also revealed that effective features of LS as a CPL methodology included opportunities for collaboration, teacher agency, opportunities for reflection, support from mentors and extended duration. A further novel feature of this research study is that it provides an expanded conceptualisation of teachers’ orientations towards the teaching of science and an in-depth account of the experience of an IKO in school based LS. Finally, the research contributes to knowledge regarding Social Learning Space (SLS) as a theoretical underpinning of school-based LS. This study highlights school-based LS as an effective CPL methodology in primary science, which is particularly pertinent at this time of curriculum change.